My Character’s arms disappear in game
I'm not really sure that this will work for you...
But weeks ago, I faced a similar problem and after some digging, I found out that some parts of my model were disappearing because I had some Un-Applied Modifiers..

Just Check the Modifier tap in the Properties panel and hit apply on any existing modifiers..

EXCEPT FOR THE ARMATURE MODIFIER!!

please report back if this worked, otherwise upload the .blend file if you can!